In some cases where the underdrain layer has sufficient depth to accommodate it, a larger bore perforated pipe (e.g. ≥ 300 mm) may be used to add further storage capacity. Ultimately this idea may result in the use of [[infiltration chambers]] to create significant reservoir storage beneath a planted area. Be sure to check with manufacturers about the compatibility of their systems with [[trees]].
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In some cases where the underdrain layer has sufficient depth to accommodate it, a larger bore perforated pipe (e.g. ≥ 300 mm) may be used to add further storage capacity to a [[bioretention]] or a [[bioswale]] project. Ultimately this idea may result in the use of [[infiltration chambers]] to create significant reservoir storage beneath a planted area. Be sure to check with manufacturers about the compatibility of their systems with [[trees]].
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===Example===
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During a 25 mm storm event, a bioretention cell receives concentrated flow from a catchment 20 times larger than its own footprint (25 x 20 = 500 mm = 0.5 m).
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The bioretention cell comprises 0.6 m filter media (K = 2.4 m/day), laid over 0.6 m clear, coarse reservoir gravel. The pipes are laid within the reservoir, 0.9 m below the surface.
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The system is designed to fill entirely during the rainstorm event. i.e. Depth to water table = 0 m.: